1. Field of the Invention
The present invention relates to a lasersensitive electrophotographic material. More particularly, the present invention relates to an electrophotographic material having an enhanced spectral sensitivity to semiconductor laser rays; i.e., over the spectrum of from red light rays to infra-red rays.
2. Description of the Related Art
Generally, a conventional zinc oxide-resin dispersion type electrophotographic material comprises an electroconductive substrate and a photosensitive layer formed on a surface of the substrate and comprises a principal component consisting of a finely divided photoconductive zinc oxide and an additional material consisting of a resinous binder and a sensitizing agent.
The zinc oxide contained in the photosensitive layer exhibits photosensitivity only at a wave length of about 370 nm located in the ultraviolet band. Therefore, in the conventional electrophotographic material sensitive to visible light rays, the zinc oxide must be presented in combination with a sensitizing coloring material in the photosensitive layer, to broaden the wave length range of light rays to which the photosensitive layer exhibits a satisfactory sensitivity.
Usually, the visible light rays are used as a photographic light for the electrophotographic material. Due to the development of various recording machines such as laser printers, however, various laser rays, for example, argon laser rays, and helium-neon laser rays, are now widely used for the electrophotographic materials. Where laser rays in the visible light band are used, it is known that the zinc oxide in the photosensitive layer is used as a photoconductive material in combination with a sensitizing coloring material, for example, Rose Bengale, Erythrosin, or Bromophenol Blue.
Now, however, semiconductor laser rays, which are in a visible or near infra-red ray band and have a large wave length of 700 to 1000 nm, are used instead of the conventional laser rays, since these semiconductor laser rays can be generated at a lower cost than that of the conventional laser rays, and can be directly modulated and used in a smaller device than that needed for the conventional laser rays.
The conventional photosensitive laser containing the zinc oxide in combination with the sensitizing coloring material exhibits a very low or substantially no sensitivity to the semiconductor laser rays, and thus the conventional electrophotographic material is substantially useless for use with the semiconductor laser rays.
Various electrophotographic materials having an enhanced sensitivity to the semiconductor laser rays are disclosed in, for example, Japanese Unexamined Patent Publication Nos. 57-46245, 58-58554, 58-59453, 59-22053, 59-78358, and 60-26949.
In those electrophotographic materials, the finely divided zinc oxide is contained in combination with a sensitizing coloring material, for example, a polymethine type cyanine dye, to extend the spectral wave length range of the usable light rays to which the electrophotographic materials are sensitive, to the long wave length side.
However, this type of conventional electrophotographic material in which zinc oxide is contained in combination with only the sensitizing coloring material, is disadvantageous in that the resultant photosensitive layer exhibits an unsatisfactory sensitivity to the semiconductor laser rays. Especially, in recording machines, for example, a laser printer, the scanning exposure is carried out at a high speed, and thus the conventional electrophotographic material containing the sensitizing coloring material is not satisfactory or practical for semiconductor laser ray exposure.
Some of the conventional electrophotographic materials sensitive to the semiconductor laser rays contain, in addition to the sensitizing coloring material, a sensitizing assistant consisting of an electron-affinitive compound, for example, benzoquinone, chloranil, phthalic anhydride, dinitrobenzoic acid or tetracyanoquinodimethane. This type of conventional electrophotographic material is disadvantageous in that the absorption of the sensitizing assistant compound on the surface of the zinc oxide particle is poor, and thus the sensitizing effect of the assistant is unsatisfactory. Further, some of the assistant compounds cause the electric resistance of the electrosensitive layer in a darkroom to be excessively decreased.